1. Technical Field
The present disclosure relates to a process for manufacturing MEMS devices. In particular, the disclosure relates to the manufacture of devices having a membrane suspended above buried cavities or channels.
2. Description of the Related Art
Hereinafter, reference will be made to the manufacture of a capacitive pressure sensor having a suspended region, also referred to as membrane, that is able to move with respect to the rest of the structure.
The disclosure is not, however, limited to this sensor, but applies also to other MEMS sensors, actuators and devices having buried channels, for instance for the use in microfluidic devices.
In particular, in the case of pressure sensors of a capacitive type, the membrane represents a variable electrode, facing a fixed portion forming a fixed electrode and separated from the latter by a buried cavity.
Various techniques are known for manufacturing the membrane, based upon gluing two substrates or removing a sacrificial layer.
For example, U.S. Pat. No. 7,273,764 describes a manufacturing process, carried out starting from a wafer made up of a silicon substrate, an insulating layer and a deposited polysilicon layer, wherein initially trenches are formed in the polysilicon layer, part of the insulating layer is removed through the trenches so as to form a cavity, the cavity and the trenches are filled with porous oxide, a covering region of porous silicon is formed on the planarized surface of the wafer, the porous oxide is removed through the covering region, and a sealing region is formed on the covering region. This process is thus rather complex on account of the operations of filling and emptying the cavity and the trenches. In addition, the resulting membrane (polysilicon layer over the cavity) is perforated and thus fragile.
In other solutions, after forming etching holes in the membrane layer and removing the sacrificial material, the holes are filled. For instance, U.S. Pat. No. 6,521,965 envisages providing the bottom electrode; forming a sacrificial region on the bottom electrode; epitaxially growing the membrane layer; forming etching holes in the membrane layer; removing the sacrificial region through the etching holes; and closing the holes with filler oxide. A similar process is described also in U.S. Pat. No. 6,527,961 to obtain pressure sensors. U.S. Pat. No. 6,012,336 uses silicon nitride or metal for filling the etching holes.
In the above processes, filling the etching holes is a critical step. In fact, it is generally not possible to use a conformal material, otherwise this would penetrate into the cavity just obtained and would bring about at least partial filling thereof, with consequent false capacitive coupling. On the other hand, the use of a non-conformal material, given also the geometrical characteristics of the holes, which are narrow and deep for the applications where a membrane of large thickness is required, does not generally enable complete closing thereof. In fact, normally the etching holes are closed near the top opening before the filling material has completely filled the holes in the bottom part.
Also the use of two different materials, a first, non-conformal, material, which restricts the top opening and prevents a second, conformal, material from penetrating into the cavity, does not solve the problem. In addition, frequently, the aim is to achieve a low pressure within the cavity, and thus for filling of the etching holes it is not possible to use materials, such as oxides, deposited at atmospheric pressure. The use of different materials is moreover not optimal since thermal or mechanical stresses can arise that worsen the electrical characteristics and duration characteristics of the finished device. The provision of thin membranes is moreover disadvantageous in the case of pressure sensors, since the thickness of the membrane is important for obtaining a more linear behavior and a higher accuracy.